This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
ACK acknowledge
BPSK binary phase shift keying
CDM code division multiplexing
CoMP coordinated multi point transmission/reception
CRC cyclic redundancy check
CSI-RS channel state information reference signal
DCI downlink control information
DL downlink (eNB towards UE)
DM-RS demodulation RS
DRS dedicated reference signal
eNB EUTRAN Node B (evolved Node B, base station/access node)
EPC evolved packet core
EUTRAN evolved UTRAN (LTE)
FDM frequency division multiplexing
ID identity
JP joint processing
LTE long term evolution
LTE-A LTE-advanced
MAC medium access control
MIMO multiple input multiple output
MU multi user
MM/MME mobility management/mobility management entity
NACK not acknowledge/negative acknowledge
OFDMA orthogonal frequency division multiple access
O&M operations and maintenance
PDCP packet data convergence protocol
PHY physical
PRB physical resource block
PCI physical cell ID
PDCCH physical downlink control channel
PDSCH physical downlink shared channel
RB radio bearer
RE resource element
Rel release
RLC radio link control
RRC radio resource control
RS reference signal
SC-FDMA single carrier, frequency division multiple access
SU single user
S-GW serving gateway
TPMI transmitted precoding matrix indicator
TTI transmission time interval
UE user equipment
UL uplink (UE towards eNB)
URS UE specific reference signals
UTRAN universal terrestrial radio access network
A communication system known as evolved UTRAN (EUTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. As presently specified the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.6.0 (2008-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8).
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The EUTRAN system includes eNBs, providing the EUTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1 MME interface and to a Serving Gateway (S-GW) by means of a S1 interface. The S1 interface supports a many to many relationship between MMEs/Serving Gateways and eNBs.
The eNB hosts the following functions:                functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
Of particular interest herein are the further releases of 3GPP LTE targeted towards future IMT-A systems, sometimes termed Rel. 10 and referred to herein for convenience simply as LTE-Advanced (LTE-A). Reference may be made to 3GPP TR 36.913, V8.0.1 (2009-03), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Requirements for Further Advancements for E-UTRA (LTE-Advanced) (Release X). Reference may also be made to 3GPP TR 36.814, v1.0.0 (2009-02), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Further Advancements for E-UTRA Physical Layer Aspects (Release X).
There is a work item in LTE-A concerning the “Enhanced DL transmission for LTE” in Rel-9 and currently specified in 3GPP. For LTE-A, the design of the downlink UE-specific reference signal in support of CoMP (sometimes also referred to as C-MIMO) and MU-MIMO is discussed. URSs (also known as DRS and referred to as DM-RS within the context of LTE-A) were agreed to be used as the demodulation reference signal in the DL of Rel-10 and Rel-9, as is detailed at the above referenced 3GPP TR 36.814 as well as 3GPP WID RP-090359, “Enhanced DL transmission for LTE” (attached to the priority document U.S. provisional patent application no. 61/247,239, filed Sep. 30, 2009, as Exhibit A). This DM-RS is only present in the assigned PRBs and for the transmitted spatial layers. It undergoes the same precoding operation (e.g. precoding vectors) as the corresponding data channel (i.e. PDSCH). The prominent benefits of PDSCH demodulation based on URS are non-constrained precoding vectors, no need for TPMI signaling in the downlink and hence reduced overhead compared to PDSCH demodulation based on non-precoded common reference signals (CRS).
CoMP and/or MU-MIMO is expected in Rel9 and Rel10 of LTE (or sometime referred as LTE-Advanced) to achieve high cell-edge and cell average throughput gains. The initialization and mapping of URS in LTE Rel-8 use a 1 subframe (1 ms) re-initialization period (sequence periodicity=one radio frame, 10 ms); the sequence is QPSK Gold which is initialized with the UE ID, the Cell ID and the subframe number; and is mapped to the URS REs in frequency-first and time-later manner in the allocated PRBs of a subframe.
Herein lays a problem. The above properties imply that the sequences transmitted from different cells will be different. However, in case of JP CoMP transmission, transmission points from multiple cells might participate in the transmission to a single, or more relevantly to the problem to multiple UEs sharing the same time-frequency resources (sharing PRBs). At detailed at document 3GPP Tdoc R1-093746, “Draft Report of 3GPP TSG RAN WG1 #58 v1.0.0 (Shenzhen, China, 24th-28th August, 2009)”, by MCC Support (attached to the priority document as Exhibit B), the CDM-based DM-RS structures were agreed for both rank 1 and rank 2 transmissions of LTE Rel9 and LTE-Advanced. In case of multi-user JP CoMP, it might happen that two users receiving their PDSCH from multiple cells but with different serving cells are SDM multiplexed into the same time-frequency resources. Considering the above decision on the CDM-based DM-RS, the dedicated reference signals of these UEs will be code multiplexed. This requires that the orthogonal RS codes of the paired UEs are different to allow for spatial multi-user interference suppression at the UE, while the overlaying scrambling code is common Therefore, the cell ID (the PCI) cannot be used in the initialization of such an RS, else there would be different RS scrambling codes for the paired UEs. On the other hand, since the length of the CDM code for DM-RS is only two, this limits the number of orthogonal RS codes to be two. One way to increase the number of orthogonal RS codes is to increase the length of CDM code which requires more resources or to use quasi orthogonal RS codes.
Relevant to these teachings are the following further documents. 3GPP Tdoc R1-093304,“Considerations on Initialization and Mapping of DM-RS Sequence”, by Nokia Siemens Networks, Nokia (attached to the priority document as Exhibit C) discusses the problem of providing a DM-RS that is invariant to the UE's ID and its PRB allocation to support CDM-based DM-RS between SDM users and/or to support MU interference tracking/suppression. 3GPP Tdoc R1-090875,“Further Considerations and Link Simulations on Reference Signals in LTE-A”, by Qualcomm (attached to the priority document as Exhibit D) notes that the URS (i.e. DM-RS) sequence should be common to all cells participating in multi-cell transmission (CoMP transmission points) to a UE (for joint transmission/processing). However, that document does not specify an exact solution. Also relevant to these teachings is 3GPP Tdoc R1-081106, “Way Forward on Scrambling Sequence Initialisation”, by Nokia Siemens Networks, Nokia, Ericsson, Qualcomm, Samsung, Panasonic, Motorola (attached to the priority document as Exhibit E).